Field of the Invention
The invention concerns a polymer composition suitable for a layer of a power cable, the use of the polymer composition in a layer of a power cable, a power cable comprising the polymer composition and to a process for producing the cable.
Description of the Related Art
Polyolefins produced in a high pressure (HP) process are widely used in demanding polymer applications wherein the polymers must meet high mechanical and/or electrical requirements. For instance in power cable applications, particularly in medium voltage (MV) and especially in high voltage (HV) and extra high voltage (EHV) cable applications the electrical properties of the polymer composition has a significant importance. Furthermore, the requirement for the electrical properties may differ in different cable applications, as is the case between alternating current (AC) and direct current (DC) cable applications.
A typical power cable comprises a conductor surrounded, at least, by an inner semiconductive layer, an insulation layer and an outer semiconductive layer, in that order.
Space Charge
There is a fundamental difference between AC and DC with respect to electrical field distribution in the cable. The electric field in an AC cable is easily calculated since it depends on one material property only, namely the relative permittivity (the dielectric constant) with known temperature dependence. The electric field will not influence the dielectric constant. On the other hand, the electric field in a DC cable is much more complex and depends on the conduction, trapping and build-up of electric charges, so called space charges, inside the insulation. Space charges inside the insulation will distort the electric field and may lead to points of very high electric stress, possibly that high that a dielectric failure will follow.
Normally space charges are located close to the electrodes; charges of the same polarity as the nearby electrode are called homocharges, charges of opposite polarity are called heterocharges. The heterocharges will increase the electric field at this electrode, homocharges will instead reduce the electric field.
Tan δ (Dielectric Losses)
The tan δ and thus the dielectric losses (which are linearly proportional to the tan δ) shall be as low as possible for both technical and economical reasons:                Low losses means that low amount of transmitted electric energy is lost as thermal energy inside the cable insulation. These losses will mean economic losses for the power line operator.        Low losses will reduce the risk for thermal runaway, i.e. an unstable situation where the temperature of the insulation will increase due to the tan δ. When the temperature is increased, normally the tan δ will also increase. This will further increase the dielectric losses, and thus the temperature. The results will be a dielectric failure of the cable that needs to be replaced.        
Compressor Lubricants
HP process is typically operated at high pressures up to 4000 bar. In known HP reactor systems the starting monomer(s) need to be compressed (pressurised) before introduced to the actual high pressure polymerization reactor. Compressor lubricants are conventionally used in the hyper-compressor(s) for cylinder lubrication to enable the mechanically demanding compression step of starting monomer(s). It is well known that small amounts of the lubricant normally leaks through the seals into the reactor and mixes with the monomer(s). In consequence the reaction mixture contains traces (up to hundreds of ppm) of the compressor lubricant during the actual polymerization step of the monomer(s). These traces of compressor lubricants can have an effect on the electrical properties of the final polymer.
As examples of commercial compressor lubricants e.g. polyalkylene glycol (PAG): R—[CxRyHz—O]n—H, wherein R can be H or straight chain or branched hydrocarbyl and x, y, x, n are independent integers that can vary in a known manner, and lubricants based on a mineral oil (by-product in the distillation of petroleum) can be mentioned. Compressor lubricants which are based on mineral oils that meet the requirements set for the white mineral oil in European Directive 2002/72/EC, Annex V, for plastics used in food contact, are used e.g. for polymerizing polymers especially for the food and pharmaceutical industry. Such mineral oil-based lubricants contain usually lubricity additive(s) and may also contain other type of additive(s), such as antioxidants.
WO2009012041 of Dow discloses that in high pressure polymerization process, wherein compressors are used for pressurizing the reactants, i.e. one or more monomer(s), the compressor lubricant may have an effect on the properties of the polymerized polymer. The document describes the use of a polyol polyether which comprises one or none hydroxyl functionality as a compressor lubricant for preventing premature crosslinking particularly of silane-modified HP polyolefins. WO2009012092 of Dow discloses a composition which comprises a HP (i) polyolefin free of silane functionality and (ii) a hydrophobic polyether polyol of PAG type wherein at least 50% of its molecules comprise no more than a single hydroxyl functionality. The component (ii) appears to originate from a compressor lubricant. The composition is i.a. for W&C applications and is stated to reduce dielectrical losses in MV and HV power cables, see page 2, paragraph 0006. In both applications it is stated that hydrophilic groups (e.g. hydroxyl groups) present in the compressor lubricant can result in increased water uptake by the polymer which in turn can increase electrical losses or, respectively, pre-mature scorch, when the polymer is used as a cable layer material. The problems are solved by a specific PAG type of lubricant with reduced amount of hydroxyl functionalities.
There is a continuous need in the polymer field to find polymers which are suitable for demanding polymer applications such as wire and cable applications with high requirements and stringent regulations.